[PATCH] powerpc/signal64: Copy siginfo before changing regs->nip

[Michael Ellerman]

Nicholas Piggin <npiggin at gmail.com> writes:
> Excerpts from Michael Ellerman's message of June 8, 2021 11:46 pm:
>> In commit 96d7a4e06fab ("powerpc/signal64: Rewrite handle_rt_signal64()
>> to minimise uaccess switches") the 64-bit signal code was rearranged to
>> use user_write_access_begin/end().
>> 
>> As part of that change the call to copy_siginfo_to_user() was moved
>> later in the function, so that it could be done after the
>> user_write_access_end().
>> 
>> In particular it was moved after we modify regs->nip to point to the
>> signal trampoline. That means if copy_siginfo_to_user() fails we exit
>> handle_rt_signal64() with an error but with regs->nip modified, whereas
>> previously we would not modify regs->nip until the copy succeeded.
>> 
>> Returning an error from signal delivery but with regs->nip updated
>> leaves the process in a sort of half-delivered state. We do immediately
>> force a SEGV in signal_setup_done(), called from do_signal(), so the
>> process should never run in the half-delivered state.
>> 
>> However that SEGV is not delivered until we've gone around to
>> do_notify_resume() again, so it's possible some tracing could observe
>> the half-delivered state.
>> 
>> There are other cases where we fail signal delivery with regs partly
>> updated, eg. the write to newsp and SA_SIGINFO, but the latter at least
>> is very unlikely to fail as it reads back from the frame we just wrote
>> to.
>> 
>> Looking at other arches they seem to be more careful about leaving regs
>> unchanged until the copy operations have succeeded, and in general that
>> seems like good hygenie.
>> 
>> So although the current behaviour is not cleary buggy, it's also not
>> clearly correct. So move the call to copy_siginfo_to_user() up prior to
>> the modification of regs->nip, which is closer to the old behaviour, and
>> easier to reason about.
>
> Good catch, should it still have a Fixes: tag though? Even if it's not
> clearly buggy we want it to be patched.

Yeah I'll add one.

cheers